The present invention relates to a fixing device having a fixing temperature switching mechanism and allowing an image which is read out from a document for instance and transferred to a sheet to be fixed to a medium while heating the medium fed by heating rollers, temperature control method for controlling the temperature of the fixing device and image forming apparatus, such as an electronic copying machine, equipped with the fixing device.
Conventionally, with an image forming apparatus, such as an electronic copying machine, a document on a document glass (document placing glass) is scanned with a light exposure lamp, a corresponding optical image is guided as reflected light to a photosensitive drum to provide a corresponding latent image there, the latent image is visualized with a toner applied by a developing device thereto, the visualized image as a toner image is transferred by a transfer device to a sheet, and the toner image on the sheet is fixed by a fixing device to the sheet.
The fixing device comprises a heating roller with a halogen lamp incorporated therein and another pressing roller and fixes a toner image to the sheet by applying temperature and pressure by these rollers to the toner-image-bearing sheet.
If, on a full-color copying machine, coping is made on an OHP sheet, sheetboard, etc., which are poor in a fixing property compared with a plain paper sheet, it is done such that each sheet passes through the fixing device at an about 50% speed-reduced rate.
This process speed reducing method results in a copying capability being reduced to about 50%, meaning that, for example, only 10 sheets can be copied in units of a minute instead of 20 sheets. Further, in order that only a fixing process is delayed with a charging, developing, transferring and separating process (pre-fixing process) kept constant, a spacing corresponding to at least an A-size sheet is required between the transferring/separating process and a nip site of the fixing device, resulting in a larger-sized apparatus.
For this reason, a temperature control method is considered by which a fixing temperature is elevated only if the OHP sheet, sheetboard or the like are to be fixed. As fixing rollers for a full-color fixing device, rubber rollers are used for high image definition. Since, however, the rubber roller is poor in heat response property than a hard roller, a time as long as 120 seconds per 10 deg. is taken for the temperature of the rubber roller to be lowered in the fixing of an image-transferred sheet at a concentration level suitable to the plain sheet after an OHP sheet has been fixed. If the roller surface temperature per se is set to be higher than even in the case of a plain sheet, the deterioration of the roller rubber is hastened, thus involving a 30% fall in the life of the roller.